J.R.R. Tolkien’s The Silmarillion was published posthumously in 1977.
Tolkien died in 1973 without completing a definitive final form of the mythology. His son Christopher Tolkien edited material from manuscripts, drafts, and revisions into a publishable volume, with assistance from fantasy writer Guy Gavriel Kay. Allen & Unwin published the book on 15 September 1977.
The Silmarillion is not a conventional sequel to The Lord of the Rings. It presents the creation of Eä, the history of the Valar and Elves, and the major events of the First Age, including the story of Túrin Turambar and the fall of Gondolin. Its publication made much more of Tolkien’s ancient mythology available to readers.